New University of Idaho research shows third-generation "granddaughter" tubers of seed potatoes exposed to glyphosate drift retain no ill effects from the herbicide.

ABERDEEN, Idaho — Bulking potatoes exposed to glyphosate drift produce low-yielding and sometimes damaged daughter crops, but the herbicide dissipates and ceases to pose a problem by the third field generation, according to new University of Idaho research.

Glyphosate is the active ingredient in products such as Monsanto’s Roundup herbicide. Several crops, including alfalfa and sugar beets, have been genetically modified to resist glyphosate, which is also popular for controlling weeds between crops.

Pam Hutchinson, a weed scientist with the University of Idaho’s Aberdeen Research and Extension Center, has studied glyphosate drift in Ranger Russets and Russet Burbanks since 2003 and is now working with Shepody plants.

For her current project, Hutchinson sprayed a Shepody crop with glyphosate at different rates and crop development stages in 2012. When potato plants were sprayed at 4-6 inches tall, Hutchinson said foliage sustained severe damage and tuber yields dropped dramatically. However, she noticed no problems in the subsequent crop when she replanted the daughter tubers.

There appeared to be little effect on yields and quality when the mother crop was sprayed with glyphosate just before vine kill.

“We thought we’d get some yield reduction and we really didn’t,” Hutchinson said.

Daughter crops sustain the most serious damage and yield losses when mother crops are sprayed at the mid-bulking stage, when glyphosate can be sucked from leaves and stored in tubers, Hutchinson said.

Hutchinson found about half of tubers from mother crops exposed at mid-bulking had symptoms of glyphosate drift, such as rough or folded skin, but even the symptomless spuds produced daughter crops with significant yield reductions.

Granddaughter tubers of mother crops exposed to all rates of glyphosate at mid-bulking, however, have produced perfectly healthy plants in this season’s test plots, Hutchinson said. She explained glyphosate degrades over time.

“It doesn’t carry over to the granddaughters. We don’t see any affect on the growth,” Hutchinson said, explaining her research demonstrates seed growers can safely sell seed after replanting at least once following glyphosate exposure.

Felt, Idaho, seed grower Dennie Arnold struggled with glyphosate drift in 2009 with seed exposed shortly before vine kill — a stage when Hutchinson didn’t notice problems. Having heard no warnings about drift and seen no restrictions on the product label, he chose to spray glyphosate to expedite ripening of wheat fields adjacent to spuds. Glyphosate affected 40-60 acres, and commercial growers complained of seed producing multiple sprouts and failed emergence.

“All it takes honestly is just a whiff,” Arnold warned.

Hutchinson has applied with the Idaho State Department of Agriculture for a Specialty Crop Block Grant, sponsored by the Idaho Potato Commission, seeking $200,000 over two years for glyphosate residue research. By measuring glyphosate residue in tubers of mother plants exposed at various growth stages and glyphosate densities, Hutchinson believes she’ll help seed growers better determine when it’s OK to sell seed affected by drift.